This invention relates to a control system for use with pneumatic actuators.
Conventional papermaking machinery for producing a continuous sheet of paper includes equipment to set the sheet properties of the paper as it is being manufactured. Generally, on-line measurements of sheet properties, such as thickness, gloss or smoothness are made by scanning sensors that travels back and forth across the width of the sheet of paper in the cross-machine direction (CD). The scanning sensors are located downstream of actuators that are controlled to adjust the sheet properties. The scanning sensors collect information about the sheet properties to develop a property profile across the sheet and provide control signals to the appropriate actuators to adjust the profile toward a desired target profile in a feedback loop. In practice, the actuators provide generally independent adjustment at adjacent cross-directional locations of the sheet, normally referred to as slices.
The actuators used in sheetmaking machinery rely on various power sources for carrying out adjustments of the actuator. Such power sources often include electric motors or hydraulic or pneumatic lines that are operated to achieved the desired control movements of the actuator. The apparatus and method of the present invention is concerned with a novel control scheme for use with actuators that rely on a source of pneumatic pressure for operation.
For example, actuators controlled by pneumatic pressure are used in steam showers that apply steam to a paper sheet passing beneath. The application of steam is used to increase the temperature of the sheet to facilitate the removal of the water from the sheet, that is to facilitate its drying to form the paper, principally by decreasing the viscosity of the water in the sheet to facilitate drainage. The steam shower is also used to improve and control the mixture and other sheet property profiles of the web by the addition of heat and water.
Applicant has developed a novel apparatus and method for control of actuators and actuator components that rely on pneumatic pressure for operation. This control scheme is based on the observation that by subjecting a component with a known starting state to a supply or exhaust pressure for a specified period, it is possible to predict the end state of the component to within an acceptable error.
Accordingly, the present invention provides a method for controlling the state of a pneumatic actuator having a valve that connects the actuator to a supply pressure and a valve that connects the actuator to an exhaust pressure, comprising the steps of:
a) closing the supply pressure and exhaust pressure valves;
b) determining the current state of the pneumatic actuator;
c) determining the difference between the current state and a desired state;
d) calculating the appropriate pressure valve to be opened and the period to be opened based on pre-determined calibration data for the actuator which permit the end state of the actuator to be predicted;
e) opening the appropriate valve for the calculated period to adjust the actuator to the desired state; and
f) repeating steps a) to e) until the current state is the desired state.
In a further aspect, the present invention provides a control unit for controlling the state of a pneumatic actuator comprising:
a valve connectable between a supply pressure and the actuator and a valve connectable between an exhaust pressure and the actuator;
a sensor to determine the state of the actuator; and
a microcontroller in communication with the sensor and the valves for determining the difference between the current state and a desired state and for storing pre-determined calibration data for the actuator whereby the calibration data allows the microcontroller to calculate the appropriate pressure valve to be opened and the period of opening so that the state of the actuator can be adjusted from the current state to the desired state.
Equipment incorporating the method and apparatus of the present invention enjoys significantly reduced manufacturing and installation costs. The size of the components and the assembly time for the equipment is also reduced. The design allows for flexibility with regard to various pneumatic component types. The system can handle different sources of feedback relating to the state of the pneumatic actuators without major changes to hardware. For example, the state of the actuators can be monitored by detecting the internal pressure of the actuator or the position of the valve portion of the actuator. In addition, feedback of the pneumatic actuator state allows for diagnosis of the system with timely scheduling of necessary maintenance. Deterioration of pneumatic components is detected and can be compensated for to maintain full operation of the system.